Under some conditions, the many individual fires created by a nuclear explosion can coalesce into one massive fire known as a "firestorm." The combination of many smaller fires heats the air and causes winds of hurricane strength directed inward toward the fire, which in turn fan the flames. For a firestorm to develop:
- There must be at least 8 pounds of combustibles per square foot.
- At least one-half of the structures in the area are on fire simultaneously.
- There is initially a wind of less than 8 miles per hour.
- The burning area is at least 0.5 square miles.
In Hiroshima, a firestorm did develop and about 4.4 square miles were destroyed. Although there was some damage from uncontrolled fires at Nagasaki, a firestorm did not develop. One reason for this was the difference in the terrain. Hiroshima is relatively flat, while Nagasaki has uneven terrain.
Firestorms can also be caused by conventional bombing. During World War II, the cities of Dresden, Hamburg, and Tokyo all suffered the effects of firestorms.